[core] Fix copy in range-based for loops

Caught by `CLANG_WARN_RANGE_LOOP_ANALYSIS = YES`.

include/mbgl/style/expression/interpolate.hpp

@@ -73,7 +73,7 @@ public:
 
     void eachChild(const std::function<void(const Expression&)>& visit) const override {
         visit(*input);
-        for (const std::pair<const double, const std::unique_ptr<Expression>&>& stop : stops) {
+        for (const auto& stop : stops) {
             visit(*stop.second);
         }
     }

include/mbgl/style/function/convert.hpp

@@ -174,7 +174,7 @@ struct Convert {
     template <typename T>
     static std::map<double, std::unique_ptr<Expression>> convertStops(const std::map<float, T>& stops) {
         std::map<double, std::unique_ptr<Expression>> convertedStops;
-        for(const std::pair<float, T>& stop : stops) {
+        for(const auto& stop : stops) {
             convertedStops.emplace(
                 stop.first,
                 makeLiteral(stop.second)